1. Field of the Invention
The present invention relates to a vibrating flow meter and method, and more particularly, to a vibrating flow meter and method for measuring temperature.
2. Statement of the Problem
Vibrating flow meters can be affected by various operational factors. One environmental factor that can affect the accuracy of a vibrating flow meter is temperature. This can include the temperature of the flow material. This can further include the temperature of the meter environment, such as the surrounding air and the conduits connected to the flow meter, for example.
A vibrating flow meter is typically designed and calibrated for operation at an expected temperature or range of temperatures. Deviation from an expected temperature or range of temperatures can affect measurements made by the flow meter. For example, the stiffness of the flowmeter structure is affected by temperature and can affect mass flow rate measurements. In addition, changes in temperature can affect a resonant frequency of the vibrating flow meter.
Temperature effects can be compensated for in the flow meter. A typical temperature compensation approach in the prior art is to affix a temperature sensor to the side of the flowmeter conduit and use a temperature measurement to scale meter output in a known manner. This can include temperature compensation for changes in elastic modulus in the meter structure due to changes in temperature, where the resonant frequency of the meter may change with temperature. The typical straight tube meter might also require a temperature sensor on the balance structure and/or the case. The difference between the balance/case temperature and the flow conduit temperature is used for compensation for thermal stress (i.e., tension or compression forces) due to changes in temperature, wherein physical dimensions of the meter may change.
FIG. 1 depicts a single conduit type vibrating flow meter 100 according to the prior art. As shown, the flow meter includes a case 103 enclosing a balance bar 102. The balance bar 102 is cylindrical and encloses conduit 101. Case 103 has end elements 104 with end faces 114 coupled by neck elements 105 to input and output flanges 106. Element 107 is the input to the flow meter; element 108 is the output. Conduit 101 has an input end 109 connected to an opening in case end 104 at element 112 which is the brace bar portion of case end 104. Brace bar portion 112 is coupled to neck element 105. On the right side, the output end 113 of conduit 101 is coupled to the case end 104 at location 112 where case end 104 joins neck element 105.
In operation, conduit 101 and balance bar 102 are vibrated in phase opposition by a driver (not shown). With a fluid flowing therein, the vibration of conduit 101 induces a Coriolis response that is detected by pick-off sensors (not shown). The outputs of the pickoff sensors are applied to electronics that processes the signals to derive the desired information pertaining to the flowing substance, such as for example a mass flow rate, a density, a viscosity, etc. The phase displacement between the pick-off sensors represents information pertaining to a mass flow rate of the fluid. A resonant frequency at either pickoff sensor represents information pertaining to a density of the fluid.
The prior art single tube meter is kept in balance over a range of fluid densities by way of a design that automatically adjusts the amplitude ratio between the flow conduit and the balance bar. This has a significant drawback in that it results in the repositioning of motionless nodes that reside along the axis of the vibrating structure. Node relocation is a problem in flow meters because the nodes are typically located on the conduit where the balance structure joins the conduit. Accordingly, the area between the nodes usually defines the active length of the conduit. The active length affects the measurement sensitivity. Further, if the nodes are repositioned, then the end portions of the tube may vibrate. This further causes the flanges to vibrate. These undesirable vibrations can further affect the measurement sensitivity.
In thermal compensation, the temperatures of different structural parts of the meter can differ in their importance to the data output of the meter. The concept of weighting the importance of a local temperature is key. If raising the temperature of the case by 10 degrees (compared to the flow conduit temperature) results in a change in indicated flow rate of 1%, and if raising the temperature of the balance structure 10 degrees results in a change in the indicated flow rate of 2%, then the balance structure temperature is said to be twice as important as the case temperature in compensating for thermal effects. The importance of the local temperature is proportional to its impact on the indicated flow rate and density. This importance of local temperatures to a meter's performance can be determined either through experiment or, as is more commonly done, through computer modeling.
In the past, temperature compensation has consisted of one temperature sensor on the flow conduit to compensate for modulus shift with temperature. A temperature sensor network comprising two or more standard temperature sensors on the balance structure and/or case has been used to compensate for thermal stress. These standard temperature sensors are usually RTDs and have a standard resistance, such as 100 ohms at zero degrees C. The resistance of RTDs increases with temperature so that the temperature of a RTD is determined from its resistance.
In a prior art thermal stress temperature compensation network, for instance, the counterbalance temperature might be twice as important for generating the output data as the case temperature. Such a meter would have two standard temperature sensors on the counterbalance and one standard temperature sensor on the case. The sensors on the counterbalance and case would be connected in series. Their resistances would thus be added. Dividing the total resistance by three gives the average resistance and thus the weighted average temperature. The result would be a temperature measurement that weighted the balance structure temperature twice as heavily as the case temperature in generating a weighted average temperature measurement for thermal stress compensation.
The thermal stress compensation network is important in straight tube meters where the change in temperature of non-tube components can put the flow conduit in tension or compression and change its frequency and sensitivity to flow. In curved tube meters, thermal stress is of less concern because the flow conduit can bend slightly to accommodate the changing dimensions of other meter components. The result is that curved tube meters show only very slight changes in frequency or sensitivity to flow due to the tensioning effects of temperature change of the non-tube components.
Single curved tube meters have another problem. They use the same amplitude-ratio balancing design as single straight tube meters. However, because the flow conduit is much less stiff, the balance structure is also much less stiff and has a much more active role in determining the vibration natural frequency. In other words, a modulus shift in the balance structure can have as large an effect on the system frequency as a modulus shift in the flow conduit. Because the frequency is fundamental in determining fluid density, and because density is necessary for compensating the flow output, it is necessary to compensate the output data for the temperature of the balance structure.
The balance structure, in its deformation during drive vibration, has areas of relatively high stress and areas of relatively low stress. The areas of high stress are more important with respect to drive frequency than the areas of low stress. The concept of importance is the same as for straight tube meters, except the straight tube meter areas of importance change the frequency by putting the conduit in tension/compression, whereas in single curved tube meters the areas of importance change the frequency through modulus shift of the balance structure.
The prior art compensation method of using multiple standard temperature sensors has drawbacks in either straight or curved tube meters. The required temperature sensor network can become complex, requiring numerous temperature sensors if the balance bar temperature importance is anything but an integer multiple of the case temperature importance. For instance, the single conduit meter shown in FIG. 1 has a case temperature that is ⅜ as important as the balance structure temperature. The prior art configuration of this network would be three temperature sensors located on the case and eight temperature sensors located on the balance structure. All eleven temperature sensors would be connected in series.
Such a solution is accompanied by drawbacks. Numerous temperature sensors are required. This results in a high overall resistance. Further, a complex circuit and numerous wires are needed. Materials costs are increased. Manufacturing costs are increased. More resistive temperature sensors increase the likelihood of wiring faults and operational failures, where one failure in a series circuit of multiple resistive devices renders the circuit inoperative. More resistive temperature sensors will likely increase the additive tolerance error.